Many current magneto-optic (MO) media constructions are four-layer (quadrilayer) thin film stacks, formed by successively depositing the individual layers on a transparent substrate. The layers consist of a first dielectric layer, a rare-earth-transition metal (RE-TM) magneto-optic layer which exhibits perpendicular magnetic anisotropy, a second dielectric layer, and a metallic reflector layer. RE-TM layers are highly reactive and must be protected from the ambient environment to achieve stability. One function of the two adjacent dielectric layers is to serve as a barrier between the MO film and surrounding environment. In addition the dielectrics must exhibit appropriate optical properties and acceptably low reactivity with the adjacent RE-TM magneto-optic film.
The rare earth-transition metal magneto-optical layer in current media is typically 20 nm to 60 nm thick. It has been suggested, however, that media structures employing a multiplicity of MO layers may be advantageous, e.g., for multi-level recording applications or mark-edge jitter reduction.
The functionality of the preceding multiple-layer MO schemes generally depends upon interaction of an appreciable fraction of the incident optical power from the laser with each MO layer. However, for a typical terbium-iron-cobalt (Tb-Fe-Co) MO quadrilayer construction, less than 5% of the incident power is present at the interface between a 30 nm thick FeTbCo layer and the adjacent second dielectric layer. Consequently, while the actual thicknesses may vary with the specific materials and media construction, it is apparent that the total MO thickness in practical multiple-layer constructions cannot become arbitrarily large. If the total MO layer thickness is limited, it is clear that the thickness of each individual layer must decrease as the number of individual layers increases. The benefits attributable to the multiple-magnetic-layer structures (e.g., increased storage density, decreased jitter, etc.) generally increase as the number of layers, N, increases; consequently, structures containing a large number of very thin MO layers are desirable.